1. Field of the Invention
The present invention relates generally to evaporative emission control systems for motor vehicles and, more specifically, to a method for controlling an evaporative emission control system for a motor vehicle.
2. Description of the Related Art
Government regulations concerning the release into the atmosphere of various exhaust emission constituents from motor vehicles are becoming increasing more stringent. As the stringency related to emissions of oxide of nitrogen, carbon monoxide, and unburned hydrocarbons, inter alia, becomes greater, it is becoming increasingly necessary to control the engine combustion process so as to avoid unnecessary instabilities. Of course, those skilled in the art know that not only engine tailpipe emissions are regulated, but also evaporative emissions. In point of fact, evaporative emission control is a very important consideration in motor vehicle design and necessitates that fuel vapor arising from the engine fuel system be drawn into the engine and burned. Because the fuel vapor can be combusted by the engine, a discontinuous flow of vapor may cause combustion instability or perhaps even engine roughness or stalling.
It is known to provide an evaporative emission control system for providing fuel vapor to an engine for a motor vehicle. An example of such an evaporative emission control system is disclosed in U.S. Pat. No. 5,816,223 to Jamrog et al. In this patent, a method is disclosed for controlling a flow of evaporative fuel vapor to an engine having a liquid fuel storage tank, a carbon vapor storage canister, and a purge system for conveying fuel vapor to the engine from the fuel tank and the carbon canister. The method includes the steps of establishing a vapor flow from the fuel tank and carbon canister through the purge system and into the engine and periodically measuring a purge system pressure within the purge system. The method also includes the steps of calculating a time rate of change of the measured purge system pressure and adjusting the flow of purged vapor to the engine in the event that the calculated time rate of change of the purge system pressure exceeds a predetermined threshold.
Since overall purge flow being drawn into the engine is relatively constant, purge air flow through the canister and vapor flow from the fuel tank are also relatively constant. If vapor flow from the fuel tank changes, air flow through the canister changes proportionally which results in a change in system operating pressure. Feed forward fuel vapor concentration change sensing strategy relies on the monitoring of the evaporative emission control system for significant, sudden changes in system operating pressure.
However, with the advent of plastic fuel tanks, undesirable system noise has been experienced with flexible wall plastic fuel tanks, which may result in false pressure changes or spikes. Also, if excessive pressure signal noise of moderate duration is present, capturing a maximum change pressure by locking in maximum and minimum pressure values when pressure trends switch direction and making pressure change measurements from these points can lead to false purge flow resets.
It is desirable to provide a method for controlling an evaporative emission control system that eliminates false pressure spikes that may be caused by flexible wall fuel tanks. It is also desirable to provide a method for controlling an evaporative emission control system that eliminates false purge flow resets which may occur if excessive pressure signal noise of moderate duration is present. Therefore, there is a need in the art to provide a method for controlling an evaporative emission control system for a motor vehicle, which meets these desires.